1. Field of the Invention
The present invention relates, generally, to fuel injector assemblies for internal combustion engines. More specifically, the present invention relates to such a fuel injector having a combined initial injection and peak injection pressure regulator.
2. Description of the Related Art
Fuel injector assemblies are employed in internal combustion engines for delivering a predetermined, metered mixture of fuel to the combustion chamber at preselected intervals. Fuel injectors commonly employed in the related art typically include a high pressure fuel passage which extends between a solenoid actuated control valve and a cylindrical bore formed in the injector body. A plunger is reciprocated within the cylindrical bore to increase the pressure of the fuel. Fuel at relatively low pressure is supplied to the fuel inlet port when plunger at its top dead center. The control valve meters the delivery of the fuel at predetermined intervals through a fuel passage to the fuel spilling port. Fuel at very high pressures is delivered to a fuel nozzle assembly and ultimately dispersed from the injector.
In the case of compression ignition or diesel engines, the fuel is delivered at relatively high pressures. Presently, conventional injectors are delivering fuel at pressures as high as 32,000 psi. These are fairly high pressures and have required considerable engineering attention to ensure the structural integrity of the injector, good sealing properties and the effective atomization of the fuel within the combustion chamber. In essence, the modern diesel engine must provide substantial fuel economy advantages while meeting ever more stringent emission regulations. However, increasing demands for greater fuel economy, cleaner burning, fewer emissions and NO.sub.x control have placed, and will continue to place, even higher demands on the engine's fuel delivery system, including increasing the fuel pressure within the injector.
In part to meet the challenges discussed above, electronic control modules have been employed to control the beginning and end of the fuel injection event, injection timing and fuel quantity, to improve fuel economy and meet emission requirements. Still, there is an ongoing need in the art for better control over additional injection parameters, such as the rate of fuel injection and peak injection pressures over the span of the injection event in a cost effective manner.
The fuel injection rate with respect to time of a conventional fuel injector is naturally a trapezoid shape having a relatively linear build-up from a low initial rate to a high rate near the end of injection. A low initial rate of injection tends to yield low NO.sub.x emissions. A high rate of injection late in the event tends to yield low particulate emission and better fuel economy.
One of the ways to lower NO.sub.x emissions and otherwise meet emission requirements is to regulate initial fuel injection rates to a lower level so that the maximum combustion temperature and, therefore, NO.sub.x formation is reduced. A short initial injection of fuel, commonly known as a pilot injection, at the beginning of the injection event has also been employed for this purpose. However, attempts to regulate the fuel injection rate at the beginning of the injection event and/or to provide pilot injections of fuel known in the related art generally suffer from the disadvantage that they are mechanically complex, require complex electronic control are only marginally effective and/or are otherwise expensive.
On the other hand, to address fuel consumption issues and improve fuel economy, it is desirable to improve the fuel spray quality. This may be accomplished by increasing the fuel injection pressure, especially at peak torque and part load. In turn, increasing injection pressure can be achieved by using an injector cam with a high velocity profile or by specifying a larger plunger diameter. However, the cam profile, plunger diameter, or other hardware configurations which provide higher injection pressures at mid-speed and mid-load usually generate extremely high injection pressures at high engine speed and high load. Such elevated injection pressures may cause serious injector reliability and durability problems. Accordingly, it is known in the related art to employ relief valves which act to limit peak system pressure. However, there remains a need in the art for a fuel injector assembly having systems which may be employed to lower the initial rate of fuel injection and to limit peak injection pressure in a simple, inexpensive and cost-effective manner.